Bullets General Manager Bob Ferry has a theory about winning in the National Basketball Association playoffs.

"The teams that do the best," he says, "Have one guy every series who comes off the bench and does the job for you. The starters can do only so much. You need an extra push from somebody."

The Bullets got that extra push from Greg Ballard in the Atlanta series, from Charles Johnson in the San Antonio round and from Larry Wright against Philadelphia.

Now the team hopes Mitch Kupchak, Plagued by a month-long shooting slump, will emerge as the catalyst during the championship round against Seattle, which opens here tomorrow.

"Mitch is important for us in this series," said Coach Dick Motta. "With him, we are so much more flexible.

"We wouldn't be here without him, remember that. He got us into the playoffs with what he did in the regular season."

The Bullets are much deeper than Seattle in the front court, an advantage they hope to exploit. But in order for them to utilize this strength completely, Kupchak has to play well.

Kupchak can be used at all three front-court spots. He also matches up well against Seattle center Marvin Webster and forward Jack Sikma. And he is four inches taller than either of the Sonics' small forwards, John Johnson or Wally Walker.

Motta wants his team to concentrate on what he calls "Seattle's belly" or inside positions. He feels that can be done best by substituting frequently for starters Wes Unseld, Elvin Hayes and Bob Dandridge.

The 7-foot-1 Webster is Seattle's only natural center. If he gets into foul troubles, Sikma, the 6-11 rookie, moves over from forward. Otherwise, the Sonics rotate 6-7 veteran Paul Silas at the three spots.

"We probably can wear them down a little that way (substituting), said Dandridge. "Our big people are strong and physical and they are going to be tough to handle for anyone."

Kupchak's game is centered around being aggressive. But he has been less effective since he lost his shooting touch in the Atlanta series.

"Thank God that we haven't lost anything because of me," he said yesterday. "That's the way I have to look at it. I'm in a slump and I don't like it but I have to think positive about the whole thing.

"I didn't shoot well midway through the season and then I came out of it. So I have to feel I can come out of this one, too."

Kupchak, who shot 57 percent last year - best ever by an NBA rookie - and 51 percent this season, has made only 31 percent of his 72 shots in the playoffs. By the sixth game against Philadelphia last week, he was aiming his shots and hesitating before releasing them. As a result, he is averaging 9.9 points, seven under his season average.

"There are two ways to handle this," he said. "You can keep away from it and hope a rest will cure it.Or you can work even harder to see if you can shoot your way out of it."

Kupchak is trying the latter. He stayed as much as an hour later than his teammates at practicee this week, taking shot after shot from either side of the basket.

"He's so darn conscientious." said Motta. He probably overreacts to everything. That's the problem. We're trying to tell him to relax and have fun and don't worry about it. He's too good a player not to come out of it with a bang.

"I'm convinced that he's going to explode one game and be a factor. Then he'll be something to watch in the series."

Until then, Kupchak admits being befuddled by a crisis "that I just don't think should be happening.

"I've stopped thinking about it. I think I was trying to get too deep and analyze things to much. That wasn't good. I couldn't relax.

"It isn't a question of some physical problem or technique problem. I'm healthy and I'm shooting the same. So it comes down to my mind. I've done it before so I know I can do it again."

Motta thinks Kupchak may be the victim of too much advice. "It's like a golfer who isn't putting well," he said. "When it's going bad, everyone wants to give advice. Everyone has an answer, but it's still up to the individual to solve the problem."

The Bullets thought Kupchak was snapping out of his slump when he scored 19 points and had eight rebounds in the fourth Philadelphia game. He missed only four of 12 shots that contest and moved with his characteristic intensity and recklessness.

"That showed me that I was still capable of playing," said Kupchak,

If Kupchak continues to have problems, Motta might turn to Joe Pace, who has appeared in only seven playoff games.

Pace, however, has played well against Webster since they were freshmen at Maryland-Eastern Shore and Morgan State, respectively. Motta feels Pace can be competitive against the Seattle center. As proof, he points to Washington's late-season victory over the Sonics here, when Pace played all the fourth quarter and was a factor in the triumph.

"Joe likes to play against Marvin," said Motta. "He's putting out in practice because I think he wants to play pretty badly. When he puts his mind to it, he can be something. He's always had the talent. It's been a matter of utilizing it correctly."